mISDN: Fixed hardware bridging/conference check routine of mISDN_dsp.ko.

In some cases the hardware bridging/conference (2-n parties) was selected,
but still pure software bridging/conference was used.

Signed-off-by: Andreas Eversberg <jolly@eversberg.eu>
Signed-off-by: Karsten Keil <keil@b1-systems.de>
Signed-off-by: David S. Miller <davem@davemloft.net>